This invention relates to an ink jet printer and a method of managing the ink quality of such printer.
In an ink jet printer using the deflected continuous jet principle, the ink not used for printing is recycled. However, the recovered ink does not have the same properties as the ink emitted during the jet, mainly because of solvent evaporation.
Two documents, referenced as [1] and [2] at the end of the specification, describe methods of controlling ink quality drift. Indeed, solvent evaporation must be compensated by adding exactly the amount of evaporated solvent to keep ink quality constant. In order to ensure feedback control of this solvent addition without fluctuation (hunting), evaporation speed has to be taken into account.
In prior art, various types of feedback control (proportional, proportional-integral, proportional-integral-derivative, . . . ) can devise a solvent addition decision by affecting a weight distribution relating to:
the present situation, or instantaneous difference between the desired value and the present operating pressure (proportional term);
the past situation, e.g. by taking into account the differences recorded for recent operating hours (integral term);
the future situation, or rather the trend of the present situation (derivative term).
These various types of feedback control are well adapted for managing ink quality. In particular, a wise choice of the relative weights of the various terms allows to improve speed and stability while avoiding oscillation (or xe2x80x9chuntingxe2x80x9d). The control principle of such feedback controls is well known to those skilled in the art.
The document referenced as [1] takes into account the measurement of the time the ink jet takes for draining a calibrated volume. A temperature sensor allows to take into account the natural influence of temperature on ink quality. Indeed, temperature has an effect on the ink""s viscosity and density. The implemented feedback control uses the draining curve as a function of temperature. A reference point is set when the machine is started to account for dispersions among the various applications envisaged. However, such a method is only a rough one. Indeed, the theoretical analysis performed in this document [1] presumes independence of the ink""s viscosity and density parameters, which is not correct: in fact, 80% of a printer""s operating pressure is associated with ink density, so that even a small variation of this density may not be neglected with regard to the evolution of the pressure term due to viscosity. In addition, to keep ink quality constant, this document [1] considers a constant operating pressure. However, such constant pressure does not ensure constant jet speed over a wide operating range. Therefore, this method is restricted to a limited range of temperature evolution at the calibration point. In practice, a float is placed inside the pressurized reservoir feeding the jet (accumulator). Drainage time measurement is subject to the vagaries of the float (jamming, sticking, oscillation, . . . ). The accuracy and the repeatability of this kind of measurement are not good. Moreover, the measurement rate is very low (about ten per hour), so much so that a feedback control built with this type of detector is neither accurate nor fast.
In the document referenced as [2], the utilized machine comes with a specific device (ball viscometer) allowing to find out the ink viscosity of the machine. A viscosity/temperature curve translates the desired operating value. However, ink density evolution is by no means accounted for. This method is independent of the ink jet and does not call upon operating pressure. This machine operates at constant pressure and does not ensure constant writing quality for a wide temperature range. Moreover, such an implementation is costly because it uses a solenoid valve, a calibrated tube, a calibrated ball, detectors, tubing, etc.
Another method is described in the document referenced as [3]. It is based on the evolution of the operating pressure as a function of ink temperature by imposing a constant jet speed. This method not only ensures ink quality feedback control, but also maintains ink quality regardless of temperature, due to constant jet speed. It also performs jet speed measurement. The characteristic curve, which is the desired ink quality value, takes into account both ink viscosity and density. However, the implementation of this method requires that the difference in level between the head and the machine be known. Any error in this respect, not checked by the machine, results in a difference in ink quality and a deterioration of printing quality. In addition, this method requires operator action, and setting the reference pressure is done by varying the operating temperature of reference machines.
It is the object of the invention to compensate for the various disadvantages of the known art documents by providing a method of managing the ink quality of an ink jet printer, which by itself devises the desired operating value without any operator action.
This invention describes a method of managing the ink quality in an ink jet printer, wherein information relating to ink pressure P, temperature T, and jet speed V, and a desired pressure value curve Pconsigne as a function of temperature T and speed V is available, of the type:
Pconsigne=axc3x97xcfx81n(T)xc3x97V2+bxc3x97xcexcn(T)xc3x97V+xcfx81n(T)xc3x97gxc3x97H
H being the difference in level between the print head and the pressure transducer, xcfx81n(T) and xcexcn(T) characteristic curves of the nominal ink, a and b being characteristic values of the ink circuit and g gravity acceleration, characterized in that, when the machine is started, jet speed is varied at its nominal value and the resulting pressure P(T)=axc3x97xcfx81(T)xc3x97V2+bxc3x97xcexc(T)xc3x97V+xcfx81(T)xc3x97gxc3x97H is measured so as to determine the coefficients a, b, xcfx81(T), xcexc(T), and H, and corrective action is taken for the ink quality to make xcfx81, xcexc, and P close to xcfx81n, xcexcn, and Pconsigne to the temperature T.
In a first operating mode, five independent values of the pair (Pfonct, V) are used to determine the five characteristics a, b, xcex94P, xcfx81, and xcexc, with Pfonct axcfx81V2+bxcexcV+xcex94P, xcex94P representing the difference in level term taken to be constant.
In a second operating mode, using the jet speeds V1 and V2, the straight line (Pfonct(V1)xe2x88x92Pfonct(V2))/(V1xe2x88x92V2) as a function of V1+V2 is plotted using a linear regression, the coefficients (axc3x97xcfx81) and (bxc3x97xcexc) are obtained, then the average is calculated for the xcex94P""s associated with the set of measurements:
xcex94Pstat=1/nxc3x97xcexa31n(Pfonct(Vi)xe2x88x92axc3x97xcfx81xc3x97Vi2xe2x88x92bxc3x97xcexcxc3x97Vi).
Advantageously, the coefficients a and b are known beforehand with sufficient accuracy for a given machine configuration from measurements performed on a sample machine and are stored in the memory of each machine produced.
Advantageously, the information regarding the ink is stored in fixed memory, e.g. as the following relations, for operation at constant concentration:
xcfx81n(T)=xcfx81n(T0)*(1+xcex1xc3x97(Txe2x88x92T0))
xcexcn(T)/xcexcn(T0)=1/(1+xcex2xc3x97(Txe2x88x92T0))
with:
T: operating temperature
T0: any temperature within the operating range
xcex1: coefficient reflecting fluid dilatancy
xcex2: coefficient reflecting fluid viscosity variation.
Advantageously, the values regarding xcfx81n(T) and xcexcn(T) are tabulated as obtained from laboratory tests.
In a first alternative of a third operating mode, the ink circuit characteristics a and b are known, the parameters Pfonct, V, and T are measured, and xcex94Pi=Pfonct(i)xe2x88x92axc3x97xcfx81(Td)xc3x97Vi2xe2x88x92bxc3x97xcexc(Td)xc3x97Vi is calculated for various operating speeds,       Δ    ⁢          xe2x80x83        ⁢          P      calculxc3xa9        =            1      /      n        xc3x97                  ∑        1        n            ⁢              Δ        ⁢                  xe2x80x83                ⁢        Pi            
is obtained and
Pconsigne(T)=xcex94Pcalculxc3xa9+axcfx81(T)xc3x97V2+bxc3x97xcexc(T)xc3x97V.
In a second alternative of the third operating mode:
xcex94Pcalculxc3xa9=(xcfx81rxc3xa9f(Td)xc3x97gxc3x97H)+(axc3x97V2xc3x97(xcex94xcfx81))+bxc3x97Vxc3x97(xcex94xcexc))
is obtained, with:
xcfx81rxc3xa9f(T): reference ink density
xcexcrxc3xa9f(T): reference ink viscosity
xcfx81encre(T): utilized ink density
xcexcencre(T): utilized ink viscosity
xcfx81encre(T): xcfx81rxc3xa9f(T)+xcex94xcfx81
xcexcencre(T): xcexcrxc3xa9f(T)+xcex94xcexc
Advantageously, the information on utilized ink characteristics is contained in an electronic tag associated with the ink container. The values of xcex94xcfx81 and xcex94xcexc can then be calculated and allow the value of the difference in level H (only unknown value remaining from the equation of xcex94Pcalculxc3xa9) to be calculated precisely. These values (xcex94xcfx81, xcex94xcexc) reflect the difference between the reference ink and the ink actually used by the machine. Relevant (xcex94xcfx81, xcex94xcexc) values calculated both during when the printer is started for the first time and for successive restarts can highlight an ink destabilization problem, it is then appropriate to inform the user of the problem observed.
In a third alternative of the third operating mode, the difference in level is known (Hconnu), the determination of the desired pressure value then being trivial.
Pconsigne=axc3x97xcfx81n(T)xc3x97V2+bxc3x97xcexcn(T)xc3x97V+xcfx81n(T)xc3x97gxc3x97Hconnu.
A specific instance of knowing the difference in level is a zero difference in level. This case is interesting for determining the hydraulic characteristics of a machine. In the latter case, measurement of the ink temperature T0 as well as several measurements of the pair (Pfonct, V) are carried out by performing a jet speed scan, the ink flowing from the jet is retrieved and a measurement of (xcfx81(T0), xcexc(T0)) is performed for this ink, then (Pfonct)/V is plotted as a function of V, the best straight line reflecting the distribution of the pairs (Pfonct/V, V) in the diagram (Pfonct/Vxe2x88x92V) is selected, the coefficient b is obtained by dividing the y-ordinate at the origin of the straight line by the measured viscosity xcexc(T0) of the ink and the coefficient a by dividing the slope of the straight line by the measured density xcfx81(T0) of the ink.
Advantageously, the same pressure transducer for determining the desired value and for measuring the operating pressure, and a temperature sensor located in the print head, are used.
Advantageously, a programmable efficiency condenser is used, by varying the condenser power supply period.
Advantageously, the same operating mode is used each time the machine is restarted, ink quality drifts are monitored, and the user is informed of any abnormal evolution thereof.
This invention also relates to an ink jet printer comprising a recovery reservoir, solvent and ink adding devices driven by a control member via solenoid valves, pressure transducers, temperature and jet speed sensors, at the output of the print head, connected to said control member, an electric control pressure regulator and an electric control condenser, both driven by the control member, and condenser power supply modulation means.
Therefore, the inventive method uses the relation linking operating pressure and ink quality. In order to obtain virtually invariable print quality for the whole operating temperature range of the machine (typically from 0 to 50xc2x0 C.), operation takes place at constant jet speed. The pressure required for obtaining this jet speed is compared with a reference pressure. This difference in current operating pressure and reference pressure reflects the evolution of ink quality. Advantageously, the inventive method allows to set the reference pressure based on information contained in fixed memory and on a start-up sequence consisting in jet speed scanning, and measuring the various associated pressures. Thus, this reference pressure is set autonomously by the machine, which has the following advantages:
Reference pressure setting is not done, as with prior art devices, by varying the operating temperature of reference machines, as the means for such an operation are considerable and expensive, in addition, the dispersion of head losses from one machine to another as well as the difference among the utilized sensors being measuring error sources.
As the operating pressure and reference pressure are the result of measurements performed with the same transducer, all the differences associated with transducer non-repeatability is done away with.
The information characterizing the machine""s hydraulics and required for setting the reference pressure can be obtained at any operating temperature because these characteristics are temperature independent.
The information characterizing the ink and required for setting the reference pressure, provided by the ink formulator, is obtained from laboratory measurements.
Partial use of the method allows stand-alone operation of the machine, which computationally determines the difference in level between the ink circuit portion and the print head.
The method is particularly well suited to circuits for which repeatability of hydraulic nozzle characteristics is ensured, e.g. using nozzles obtained through electrodeposition, electrodischarge machining or laser drilling. The machine then produces the formulator""s ink from the ink contained in the reservoir (ink cartridge). This advantage is considerable from the industry""s point of view because the tolerances associated with ink production are extended. Thus, industrial ink production is made easier without penalizing machine operation. Moreover, the consistency of operator information and machine calculation is controlled by the machine""s software. The major risk of sign errors for this head/circuit difference in level is thus avoided.
In case of ink type modification (color, type), all that has to be done is replace the characteristics of the old ink with those of the new one in order to be operational.